=(($a$1 - 25569) * (60 * 60 * 24)) - (60 * 60 * 9)
"2019/9/30 23:59:59" -> 1569855599
=25569 + (($a$1 + (60 * 60 * 9)) / (60 * 60 * 24))
1569855599 -> "2019/9/30 23:59:59"